1. Field of the Invention
The present invention relates to a ball screw rack assist type electric power steering apparatus.
2. Related Background Art
Steering systems of automobiles widely use a so-called power steering apparatus for assisting a steering operation by use of an external power source. The power source for the power steering apparatus has hitherto involved the use of a hydraulic pump, and this hydraulic pump is driven by the engine in many cases. This type of power steering apparatus has, however, a large power loss (on the order of several through ten horsepower (HP) at a maximum load) of the engine because of driving the hydraulic pump at all times and is therefore hard to apply to mini-sized motor vehicles exhibiting small displacements. Even the automobiles having comparatively large displacements, when installed, come to have such an inevitable demerit that their running fuel economy becomes low enough not to be negligible.
Such being the case, an electric power steering (which will hereinafter be abbreviated to EPS) involving the use of an electric motor as a power source has attracted the attention over the recent years for obviating those problems. The EPS apparatus exhibits characteristics, wherein there is no direct drive loss of the engine because of using a battery loaded into the car as a power source for the electric motor, a decrease in the running fuel economy is restrained as the electric motor is started only when assisting the steering operation, and in addition the precise electronic control can be extremely easily performed.
On the other hand, for a steering gear mechanism for a passenger car, a rack-and-pinion steering gear is mainly employed at the present, since the rack-and-pinion gear has high rigidity performance and low in weight. Then, the EPS apparatuses for the rack-and-pinion steering gear include a column assist type in which the electric motor is arranged sideways of the column in order to drive a steering shaft and a pinion itself, and in addition a ball screw rack assist type in which a rack shaft is driven by an electrically-driven ball screw mechanism. In the ball screw rack assist type EPS apparatus (which will hereinafter be simply called the rack assist type EPS apparatus), the assist force does not act on a meshing surface between the pinion and the rack, and therefore a contact surface pressure between these two members, which is to be a factor for abrasion and deformation as well, is relatively small.
According to the rack assist type EPS apparatus, a ball screw shaft male thread groove formed in a rack shaft engages with a female thread groove formed in a ball nut through a multiplicity of circulation balls (steel balls), and the ball nut is rotated by the electric motor, thereby moving the rack shaft in the axial directions. A power transmission method for transmitting the power from the electric motor to the ball nut may be a timing belt system disclosed in Japanese Utility Model Publication No. 6-49489 etc, however, a general method is a gear system disclosed in Japanese Utility Model Publication Nos. 5-14939 (a first prior art) and 2-46455 (a second prior art) etc. In case of the conventional rack assist type EPS apparatus, if low of an assembly accuracy of the respective parts constituting the ball screw mechanism and the rack and pinion mechanism, a misalignment occurs between the ball nut and the rack and pinion pair. In this case, so-called distortions are formed between the ball nut and the rack shaft, with the result that a smooth movement of the rack shaft is hindered. This hindrance of the smooth movement might cause a loss of driving force, an abnormal abrasion and, besides, noises when operated. Further, if the working accuracy and the assembly accuracy of the housings that accommodate and hold the ball screw mechanism and the electric motor, are low, a backlash occurs between the gear pair connecting the ball nut and the electric motor together, and the noises are likewise emitted.
Such being the case, each of the rack assist type EPS apparatuses according to the first and second prior arts adopts an aligning mechanism for supporting the ball nut so as to be swayable about the housing through a spherical bearing in order to obviate a misalignment between the ball nut and the rack shaft. Large increases in the number of manufacturing processes and in the costs for the product are, however, inevitable for providing the spherical bearing between the housing and the ball nut. Moreover, the spherical bearing utilizes the slide on between the metal members, and hence, if a comparatively large load acts, a predetermined aligning operation is not obtained due to a rise in slide friction force, with the result that the misalignment is not obviated in the great majority of cases. Further, if the ball nut is supported on the spherical bearing, a central position of the ball nut remained unchanged, so that the backlash between the gears is not, as a matter of course, canceled.
Generally the ball nut configuring the ball screw mechanism is structured so that a rolling bearing (a double-row angular ball bearing etc) is fitted on and made integral with the ball nut body. The ball nut is fixed by a ring bolt or a C-shaped stopper ring brought into contact with an outer ring of the rolling bearing within the housing.
In the conventional rack assist type EPS apparatus, the C-shaped stopper ring comes off and falls out of an engagement groove formed in the housing, or the ring bolt slackens and falls out due to a careless operation in the assembly and vibrations when driving. In this case, since the ball nut moves in an axial direction by dint of an external force, a proper axial force can not be given to the rack shaft. This results in an extremely unagreeable feeling of the steering and in an impossibility of performing the precise steering.
According to the rack assist type EPS apparatus adopting the gear type power transmission method, an idle gear is interposed between the gear on the electric motor and the gear on the ball nut in order to avoid an increase in diameter of each of these two gears.
In the rack assist type EPS apparatus, the gear train for the power transmission, the ball screw mechanism and the bearing for supporting the ball nut, have hitherto been lubricated by respective lubricants (normally greases) for exclusive uses therefor. Hence, in this type of EPS apparatus, three or four pieces of shaft seals are attached to side portions etc of the gear portion of the ball nut, thereby preventing the lubricants from being mixed. If many shaft seals are thus used, a large friction loss occurs when the ball nut rotates, there are caused a loss of driving torque and an unagreeable feeling of the steering. Further, the steering gear case and the ball nut need high-precision working of seal seats and seal slide surfaces in order to accommodate and hold the shaft seals. This is a factor for increasing the costs for the product along with the rise in the number of components.
It is an object of the first invention of the present application to provide a rack assist type electric power steering apparatus capable of improving an operability and canceling a backlash etc in a gear train of a driving system.
To accomplish the object given above, according to the first invention, an electric power steering apparatus comprises a ball nut rotationally driven by an electric motor through a gear train and having a female thread groove formed in the inner periphery thereof, a rack shaft disposed through said ball nut on the axis thereof and having a male thread groove so formed in its outer periphery as to face to the female thread groove of the ball nut, a ball screw mechanism having a number of circulation balls interposed between the female thread groove and the male thread groove, and a housing for holding the ball nut rotatably through the bearing, wherein elastic members are interposed between the housing and the ball nut in order to permit the ball nut to make a predetermined displacement.
According to the first invention, even if a misalignment occurs between the ball nut and the rack and pinion pair, the ball nut displaces with a deformation of the elastic member, thereby obviating the misalignment. Further, a backlash between the gears due to a defect in working of the housing and so on, is also canceled by biasing the ball nut in a direction right-angled to the axis with the elastic member.
It is an object of the second invention of the present application to provide an electric power steering apparatus capable of preventing a fall-out of a fastening element for fixing the ball nut and thus preventing a decline of a steerability.
To accomplish this object, according to the second invention, an electric power steering apparatus comprises a ball nut rotationally driven by an electric motor and having a female thread groove formed in the inner periphery thereof, a rack shaft disposed through the ball nut on the axis thereof and having a male thread groove so formed in its outer periphery as to face to the female thread groove, a number of circulation balls interposed between the female thread groove and the male thread groove, a housing for holding the ball nut rotatably, and a fastening element for fixing the nut to the housing, wherein the electric power steering apparatus is provided with a fall-out preventing mechanism for preventing the fastening element from falling out off within the housing.
According to the second invention, the fastening element does not come off the housing, and hence, even if the fastening element comes off and slackens, the ball nut does not move in excess of the predetermined range.
Further, in the electric power steering apparatus according to the second invention, the housing may be constructed of a first housing accommodating the ball nut and the fastening element and a second housing fixed to the first housing, and the fall-out preventing mechanism may be a fastening element contact member, formed on the second housing, for regulating the fastening element from moving in the fall-out direction. With this contrivance, the fall-out preventing mechanism is formed simply by fastening integrally the first housing and the second housing to each other.
Moreover, in the electric power steering apparatus according to the second invention, the fastening element may be a stopper ring fitted to the first housing, and an interval between the stopper ring and the fastening element contact member may be, in an assembled state, set smaller than a thickness of the stopper ring. This scheme prevents the stopper ring, even if on the verge of coming off due to a careless operation etc when assembled, from completely falling out because of coming into contact with the fastening element contact member.
In the electric power steering apparatus according to the second invention, the fastening element may be a ring bolt helically fitted to the first housing, and an interval between the ring bolt and the fastening element contact member may be, in an assembled state, set smaller than an effective helical-fitting length of the ring bolt to the first housing. With this contrivance, the ring bolt, even when slackening due to the vibrations etc during driving, comes into the contact with the fastening element contact member and is thereby prevented from completely falling out.
It is an object of the third invention of the present application to provide a rack assist type electric power steering apparatus capable of reducing a friction loss when operating and the number of components.
To accomplish the above object, according to the third invention, a rack assist type electric power steering apparatus comprises a ball nut rotationally driven by an electric motor, a ball screw mechanism interposed between the ball nut and a rack shaft, and a bearing for rotatably supporting the ball nut, wherein the ball screw mechanism and the bearing share a lubricant with each other.
Further, in the rack assist type electric power steering apparatus according to the third invention, it is preferable that a power transmission mechanism be interposed between the electric motor and the ball nut, and the power transmission mechanism, the ball screw mechanism and the bearing share the lubricant with each other.
In the rack assist type electric power steering apparatus according to the third invention, the power transmission mechanism may be a gear type power transmission mechanism or a chain type power transmission mechanism or a belt type power transmission mechanism or a friction roller type power transmission mechanism.
According to the third invention, there is no necessity for the shaft seal, whereby the friction loss caused by the rotations of the ball nut decreases, and the enhanced feeling of the steering is attained. Besides, the decreases in the number of working processes for the steering gear case etc and in the number of components, lead to the considerable reduction in the costs for the product.